I will give you 3 arguments very simples that disassemble the flat Earth. If after reading this post someone keep thinking in the flat Earth, he can left a comment whis their arguments.
We will start for the easiest argument. The flat Earth theory defends that no matter where you are on the Earth you will see the same stars, would only change the distance. But really, if we use telescopes from differents places on Earth in the same moment, the sky we will see would be different.
The second one is so similar. For a flat Earth theory's defender the nocturn sky would rotate in the same way in all places of the flat. What really happens is that for someone in southern hemisphere the stars will rotate in the anti-clockwise direction, meanwhile for a observer in the northern hemisphere they will rotate in te clockwise direction.
The last argument that I will give it's related to seismic waves. Lehmann used seismic waves P and S to establish that the Earth have a center. P waves propagate in both solid and liquid media, while S waves propagate only in solid media. Studying the propagation of this waves in differents parts on Earth Lehmann found that so that the data would be conceivable the Earth must be spherical and with a liquid core.
Lehmann's studies are so bigger, and really interesting, but if we understand this It's enough to prove the Earth is spherical. If you want, in another post I will explain harder the Lehmann theories.
PD: this post is a translation from my post: https://steemit.com/spanish/@rccc/por-que-la-tierra-no-puede-ser-plana